The present invention relates to composite allografts used in orthopedic surgery, and in particular to a composite acetabular allograft cup, to a method and apparatus for forming the composite acetabular allograft cup, and to a method of using the composite acetabular allograft cup in hip replacement surgery.
There is a need for methods of replacing or strengthening certain types of bone defects; for example, in the case of hip replacement surgery. A hip joint is a ball and socket joint in which the ball is the femoral head and the socket is called the acetabulum (due to its supposed resemblance to a vinegar cruet). The cavity of the acetabulum is formed from three parts of the pelvic bone: above by the ilium, behind and below by the ischium, and internally by the os pubis. Patients who are otherwise candidates for hip replacement surgery may have acetabular defects. The acetabulum may for various reasons, including disease, trauma or prior surgery, contain defects such as missing or eroded portions of the acetabular wall. These defects must be corrected or compensated if the surgery is to be successful.
In hip replacement surgery, a hip joint prosthesis, comprising a femoral component and an acetabular component, is employed to replace the femoral head and the acetabulum. The acetabular component may include a hemispherical metal cup or ring and a low-friction plastic liner of ultra-high molecular weight polyethylene. The procedure may also be done without the metal cup, using only the liner which is cemented in place.
One method of dealing with an acetabular defect is to repair the defect with a bone graft (either an allograft, typically harvested from a cadaver, or an autograft from the patient's own bone tissue). Due to the significant weight bearing role of the hip joint, the stability and strength of the bone graft is a major concern. Metallic support cups may be required to support the bone graft material as disclosed in MacCollum (U.S. Pat. No. 4,904,265). MacCollum discloses a support cup in the shape of a rigid metallic hemisphere with a flange to support the bone graft. The outer surface of the support cup is disclosed to be porous to support bone ingrowth. A bearing insert of low friction material for receiving the ball of the femoral prosthesis is mounted within the support cup.
As an alternative to bone grafts, Grimes (U.S. Pat. No. 5,176,711) discloses an acetabular hip prosthesis in which the acetabular component of the prosthesis includes an augmentation piece to fill a rim or cavitary defect. Likewise, Collazo (U.S. Pat. No. 5,326,368) discloses a modular prosthetic acetabular cup to provide various cross sections as desired to fill acetabular defects.
Another method of remedying an acetabular defect is disclosed in "Bone Grafting in Total Hip Replacement for Acetabular Protrusion" by McCollum, et al., Journal of Bone and Joint Surgery, Vol. 62-A, No. 7, 1065-1073 (October 1980). The McCollum article discloses the use of wafers of bone to fill a defect in the acetabular wall.
A slightly different technique is disclosed in "Bone Grafting in Total Hip Replacement for Acetabular Protrusion" by Slooff, et al., Acta Orthop. Scand, 55, 593-596, (1984). While Slooff et al. disclose the use of a bone graft to close an acetabular defect, Slooff et al. also disclose surrounding the graft with a wall of cancellous bone chips which are molded and impacted by using the socket trial prosthesis. (Cancellous bone has a spongy or lattice-like structure and may be derived from cadaverous bone tissue such as femoral heads.) Slooff et al. disclose a technique of repairing an acetabular defect in which cancellous bone chips are molded and impacted around a bone graft, but do not disclose the addition of cement to the impacted bone chips.
Gie, et al. in "Impacted Cancellous Allografts and Cement for Revision Total Hip Arthroplasty", The Journal of Bone and Joint Surgery, Vol. 75-B, No. 1, 14-21 (January 1973) disclose the use of impacted cancellous allografts and cement for fixation of the femoral component in total hip arthroplasty. The technique disclosed by Gie et al. involves packing allograft bone chips into the femoral canal using the trial femoral component. The chips are repeatedly impacted after which cement is introduced and pressurized to force the cement into the graft. Pressure is maintained until the cement has sufficiently solidified. While Gie et al. disclose impacting cancellous bone chips into the femoral canal after which cement is added to the impacted bone chips and pressurized to force the cement into the graft, Gie et al. do not disclose the use of this technique in relation to the acetabulum. Neither Slooff et al. nor Gie et al. disclose the formation of a composite acetabular cup outside the body of the patient prior to surgery.
It is known to form human tissue into particular shapes to create desired natural tissue grafts. For example, U.S. Pat. No. 4,678,470 issued to Nashef et al. on Jul. 7, 1987 for "Bone-Grafting Material" discloses a bone grafting material derived from allogenic or xenogenic bone which may be machined into a predetermined shape.
U.S. Pat. No. 5,329,846 issued to Bonutti on Jul. 19, 1994 for "Tissue Press and System" discloses a press for shaping or compressing a piece of tissue by the movement of two members relative to each other. Various shapes of the two movable members may be selected so as to produce tissue in the desired shape. While the Bonutti invention is primarily directed to the compression and shaping of soft tissue, portions of the disclosure suggest the shaping of bone tissue with the addition of polymeric material (column 11, lines 11-13). Bonutti does not expressly disclose the formation of an acetabular cup using cancellous bone chips and cement. Furthermore, the Bonutti press does not disclose a press of the rack-and-pinion type. While Bonutti discloses the importance of monitoring and controlling the pressure applied to the compressed tissue, it is in the context of maintaining graft tissue in a living state to improve graft viability and tissue healing. In this context Bonutti discloses the use of pressure sensors and force-limiting means such as the mechanism found on torque wrenches. FIG. 6A of Bonutti discloses such a torque limiting mechanism and a pressure gauge.
Rack-and-pinion gearing and load switches are known in the art of manual presses used by machinists and in manufacturing environments. For example, the common arbor press may operate by means of a manual lever through rack-and-pinion gearing. See, for an example in an unrelated art, U.S. Pat. No. 3,686,922. Likewise, U.S. Pat. No. 3,741,706 issued to Conley, et al. on Jun. 26, 1973 discloses a molding device for forming a shaped object (a toy) from a soft moldable material. A manually operated lever acting through a pair of rack-and-pinion gear mechanisms is used to move one part of a mold against the other half of a mold to mold a three dimensional object. It is known to use pressure gauges, load limiting devices and the like in presses in the manufacturing environment. An example is U.S. Pat. No. 3,786,676 which discloses a compression testing machine having an in-line load cell.